It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "None of you should ever say: 'I have forgotten such-and-such a verse,' for he did not forget it himself; rather, he was made to forget it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10399